Conventional Fixtures Plano-Convex Fixtures
- Thread starterLXPlot
- Start date
jglodeklights
Well-Known Member
You'd be surprised. Most ETC S4's are actually more similar to a PC fixture than you would think. 19, 26 and 50 degree units all use a simple single glass PC lens in place of the more complicated 2 lens design utilized in older units. Because of modern optical engineering, lamp type and reflector design, field illumination evenness suffers in comparison to projection sharpness. This makes sense given the intended purpose of the S4. However, like a PC unit, field angle can be changed by altering the distance between lens and lamp, aka focusing the lens tube.
If you wanted, you could take a piece of metal, put it in a PC between the lens and lamp at the right point, focus it properly and get a somewhat discernible cut. The advantage really lies, though, in being able to get a sharp circle or a nice even wash with the PC units, in a compact package.
Theories as to why we don't use them in the US much? Like a standard Fresnel, they transmit less light onto the stage per lamp lumen than a PAR of any type. Less reflective surfaces surrounding the lamp with more non reflective space to convert light to heat. Also, here in the US we like nice compact packages that allow some shaping of the light more so than just a circle. We seem to have an obsession with creating oblong shapes.
If you wanted, you could take a piece of metal, put it in a PC between the lens and lamp at the right point, focus it properly and get a somewhat discernible cut. The advantage really lies, though, in being able to get a sharp circle or a nice even wash with the PC units, in a compact package.
Theories as to why we don't use them in the US much? Like a standard Fresnel, they transmit less light onto the stage per lamp lumen than a PAR of any type. Less reflective surfaces surrounding the lamp with more non reflective space to convert light to heat. Also, here in the US we like nice compact packages that allow some shaping of the light more so than just a circle. We seem to have an obsession with creating oblong shapes.
JChenault
Well-Known Member
I beg to differ - but first some definitions.
A PC lens stands for Plano Convex. IE it is flat on one side and curved on the other.
A traditional PC fixture has a lamp with a spherical reflector and a single PC lens. You can change the size of the beam by moving the lamp closer and further from the lens - but there is no way to move the beam soft to hard. The quality of the beam gets pretty bad when you set the unit to a small spot as you tend to project an image of the filament instead of a nice round beam. You can place something like a gobo right behind the lens and shape the beam a bit. In the old, OLD days - you had fresnels and PC's and you used a PC when you needed a sharp edge and a longer throw.
The Elipsoidal Reflector spot was a vast improvement over the PC. It was more efficient, you could put a gobo in the gate and make a sharper projection, and you could focus the image to be sharp or fuzzy. You could not change the size like you could with a PC - but the Elipsoidal ( AKA Leko) pretty much trumped the PC.
Any elipsoidal is as much like a PC fixture as a resistance plate dimmer is like an SCR dimmer. They both have wire and electricity but they are not the same beast. (IMHO)
The use of multiple PC lenses in some fixtures is no more than a way of getting the same effect as a single lens with more curvature. I believe if you look closely at the S4 fixtures you will discover that in addition to a different reflector design, they do not use PC lenses ( they are curved on both sides of the lens). Additionally some of them have multiple lenses.
So to recap - a hard edged long throw PC unit is a pretty lousy fixture. It does not blend well, and is very inefficient. At some zoom ranges you get a really awful beam. The Elipsoidal was far superior.
Now you also had smaller PC units that were not intended for long throw and would be used more like a fresnel. However you can get a much shorter focus lens with a fresnel lens than you can with a PC lens ( there is less glass to heat up and crack). A Fresnel fixture was basically a PC style fixture with a fresnel lens instead of a PC lens. The Fresnel was a bit more efficient because you could get the lamp closer to the back of the lens ( more light from the lamp hits the lens) and still get a small enough spot to be useful.
I did see a Selecon PC a number of years ago - but it had a special lens with a pebble back to soften the beam and they were intended to be used more like a fresnel. It made no sense to me why I would use a PC over a fresnel at the time.
I had the Royal Ballet of Flanders at my space twice, many, many years ago and at the time they were touring with a 240v rig. The backlighting was all 2kw PC Beams, similar to this:
ADB-TTV Technologies
The punch was incredible and they would use 6 in a color system to cover a stage 40x35, 2 Electrics worth as diagonal L&R and one filling up center per electric. An amazing look at the time, when we were using Par64 wides in a 3x4 lamp wash. The PC Beam was much more defined look, especially as they could do diagonals.
That said, I'd bet I could come close to duplicating this with S4 25/50 zooms at 750w, in fact have done so.
So in reality, a good quality modern zoom ellipsoidal can do the same thing, only project gobo images and do shutters as well.
Thus nobody makes PC Spots
ADB-TTV Technologies
The punch was incredible and they would use 6 in a color system to cover a stage 40x35, 2 Electrics worth as diagonal L&R and one filling up center per electric. An amazing look at the time, when we were using Par64 wides in a 3x4 lamp wash. The PC Beam was much more defined look, especially as they could do diagonals.
That said, I'd bet I could come close to duplicating this with S4 25/50 zooms at 750w, in fact have done so.
So in reality, a good quality modern zoom ellipsoidal can do the same thing, only project gobo images and do shutters as well.
Thus nobody makes PC Spots
JChenault
Well-Known Member
I think you are mixing terms. A PC lens is a standard part of many fixtures. Many ellipsoidal have PC lenses in them. A PC fixture has a spherical reflector and a PC lens. I can't think of any semi modern architectural fixtures that are a simple spherical reflector and PC lens. The spherical reflector is not very efficient for getting light out of a lens.
All things considered, a fresnel and a PC spot are really the same. The optical principles used in each are the same and the quality of light that you get out of them is very similar. The only difference is the lens design. In theory, if you were able to get the correct PC lens you could probably just swap it into a fresnel body (I think we had a thread on this at some point). As stated earlier, both fixtures have a spherical reflector and the lenses were originally based on spherical lenses.
At the time of their invention the Fresnel lens and the step-lens were improvements over the PC lens. As the technology of lenses got better we were able to take a PC lens and essentially compress it into either a step lens or a fresnel lens. If you were to take a fresnel lens and stack all the rings you would end up with a PC lens There are some modern fixtures like the 10˚ source four and the HES Showgun that use micro-fresnel lenses and are perfectly capable of achieving a super sharp beam.
Most modern fixtures are using ashperic lenses. We have much higher precision lens manufacturing today that allows for grinding optics into much more efficient designs. This is why many source four lens tubes have only one lens. The more precise the glass and the less of it you have, the more light you get.
At the time of their invention the Fresnel lens and the step-lens were improvements over the PC lens. As the technology of lenses got better we were able to take a PC lens and essentially compress it into either a step lens or a fresnel lens. If you were to take a fresnel lens and stack all the rings you would end up with a PC lens There are some modern fixtures like the 10˚ source four and the HES Showgun that use micro-fresnel lenses and are perfectly capable of achieving a super sharp beam.
Most modern fixtures are using ashperic lenses. We have much higher precision lens manufacturing today that allows for grinding optics into much more efficient designs. This is why many source four lens tubes have only one lens. The more precise the glass and the less of it you have, the more light you get.
The PC is still around. From what i understand it is still very common in Europe. Robert Juliat has a product that has an optional PC,Pebble, or Fresnel Lens capability. http://www.robertjuliat.fr/PDF/Documents/Doc_LUTIN.pdf It is not really common in the US though.
David Ashton
Well-Known Member
JChenault
Well-Known Member
Well - for some definition of the same. The difference of course is that a Fresnel has a fresnel lens in it - and a PC has a Plano Convex Lens in it.
If your PC lens is pebbled on the back, your PC fixture can give you a soft beam like a fresnel. In my mind, however, I don't think of a pebbled PC lens as a standard PC lens - so in my mental image ( and certainly in the antique PC fixtures I have in my house) the PC produces a hard edged beam.
If you put a PC lens of the same focal length as a fresnel lens in a fixture, I would expect that it would pretty quickly crack due to the thickness of the glass and the thermal expansion. I've never tried it but that is my expectation.
Blackfaer
Member
Yes. This is a key difference. PC instruments typically get a more controlled narrow beam than Fresnels, but would be more limited in getting as wide a flood, because the glass would be so much thicker with a wide-angle PC lens. Also, PC's light 'texture', the quality of the light, is more like an ellipsoidal, with somewhat hard-edged shadows resulting, as opposed to the soft and blurry shadows of a Fresnel. They are indeed still popular in Europe, frequently used in place of ERS units when you don't need sharp gobos or shutter cuts - like for high sides or front light washes.
Think of them as a cross-breed between an ellipsoidal and a Fresnel, and you'll have an idea of how they can be used. In Europe, for a variety of reasons, they tend not to use fixed-length lenses for their ellipsoidal lights, but use mostly zooms. As you can imagine, this means the ellipsoidal purchase prices are a bit higher than what we think of as typical in the US. As such, PC's much cheaper price point but ability to be used more effectively than Fresnels as alternatives to ERS units is a pretty major selling point. Since American designers and users tend to be more happy and comfortable with fixed-lens ERS units instead of zooms, the price differential is less than what Europeans are looking at, and so PC's didn't catch on as much.
A quick addition: in response to the original question, the main reason in the US they don't show up much is mostly because they never became popular. Without a bigger price difference or other needs to fill, US users tended to go with what they knew, either selecting a Fresnel for a comparable price, or paying just a few bucks more to get an ERS complete with gobo slots and shutters. Since they didn't get purchased very often, the younger generation didn't grow up using them, so they didn't spec them when they were purchasing lights later on... and so custom creates custom.
Last edited:
I've always felt that the story of the PC in North America is much the same as that of the footlight. While it was once a dominant technology, once better fixtures became available, as well as new lighting positions, it was shunned. It was really no fault of the PC, as the non-pebbled coke bottle glass lenses and lesser quality lamps of the day did not help its case. The PC and the footlight are not bad instruments, its simply that they gained a bad rep through the ways they were originally used.
Similar threads
